Chiral biaryl monodentate cyclic phosphonites and phosphites for asymmetric catalysis.
A serieso f chiral biaryl cyclic monophosphoniteli gands of type RP(OAr)2 (R =
Ph, Me, t-Bu, o-anisyl and Cy; (OAr)2 = 1,1'-binaphthyl-2,2'-diyl, 9,9'-
biphenanthryl- 10,1 0'-diyl) have been prepared. The ligands have been synthesised by
reaction of the dichlorophosphine precursor with the corresponding biphenol (HOAr)2.
Chiral biaryl diphosphonite ligand (ArO)2PCH2CH2(OAr)2 derived from binaphthol
has also been prepared. Platinum(II), palladium(II) and rhodium(I) complexes of these
ligands are described and the X-ray crystal structures of five of these complexes are
reported and discussed.T he electronic propertieso f the phosphonitesa re betweent hose
of phosphite and phosphine, and this is reflected in their coordination chemistry.
The new chiral biaryl monophosphites of type (Ar'O)P(OAr)2 have been
synthesised straightforwardly by condensation of a phenol and the corresponding
chlorophosphites in the presence of NEt3. Preliminary studies of their coordination
chemistry with platinum(II) and rhodium(I) have been carried out. The twist on the
seven membered P(OCCCCO) ring leads to a deshielding of the 31P signal for the
biaryl ligands and in the complexes studied here.
The asymmetric hydrogenation of a-enamides with rhodium(I) complexes of
the phosphonites and diphosphonites as catalysts is discussed. The ligands have been
screened in the asymmetric hydrogenation of two substrates: methyl-2-acetamido
acrylate and methyl-(Z)-2-acetamido cinnamate. Monodentate phosphonite systems
show, in some cases, higher enantioselectivity (up to 92% ee) than the analogous
bidentate. The results presented here challenge the long-accepted wisdom that chelating
ligands are necessary to achieve high enantioselectivities in asymmetric hydrogenation.
NMR studies and X-ray crystallographic data show that the asymmetric ligand profile
caused by the biaryl units in these phosphonites has three consequences: (i) rotation
about the M-P bond in monodentate phosphonites is inhibited; (ii) a different rotamer
for the monodentate from that in the chelate analogues is favoured; (iii) the favoured
rotamer in the monodentate causes more effective chiral induction in the hydrogenation
catalyses. The generality of these new concepts has been probed with some preliminary
catalytic studiesw ith monodentatep hosphitel igands.
The phosphonite ligands have also been tested in the copper(I) catalysed 1,4-
addition of diethylzinc to enones and nitro-olefins in an attempt to extend their potential
use in asymmetric catalysis. The phosphonite ligands show moderate to high ee's for
the ethyl transfer to enones (cyclic and acyclic). The monophosphonite ligands induce
higher enantioselectivity for acyclic enones: ee's up to 82% are the highest obtained
with a monodentate phosphonite ligand in asymmetric 1,4-addition of diethylzinc to